WhatIs the Originof Pleural Transudates andExudates

T he classic teaching has been that pleural effusions arise from the pleural capillaries. Transudates were thought to result from an imbalance of the hydrostatic and osmotic forces in the pleural capillar ies, leading to an increased flow of low-protein liquid into the pleural space, and exudates were thought to result from increased permeability of the capillaries in the pleura, leading to the accumulation of high protein liquid in the pleural space. We now believe that this theory needs to be modified to include another common source of the liquid, namely: the interstitial space of the lungs. In recent experimental and clinical studies involving hydrostatic pulmonary edema, transudates have been shown to arise from the interstitial space of the lung. When sheep were volume-loaded to cause lung edema, transudative liquid flowed across the visceral pleura of the isolated in situ ‘?� The pleural fluid contained the same protein concentration as did the lung lymph and the interstitial edema liquid in the lung. The volume ofpleural fluid constituted about 25 percent of all edema liquid formed in the lung. In other experiments with high-pressure pulmonary edema in sheep, pleural fluid accumulated only after